The present invention regards a process for determining the actual position of a rotation axis of a transportation mechanism, in particular a wafer transportation mechanism with at least one rotation element, rotatable around a rotation axis and at least one transportation arm mounted to the rotation element. The invention furthermore regards a process for aligning a transportation mechanism through a determination process according to the invention.
During the manufacture of electronic components like memory chips, microprocessors, customer specific circuit designs, or logical components, a multitude of various semiconductor technology processes are being used. These processes have to be very well and precisely adjusted to each other and often have to allow the production of a multitude of components with reproducible properties per unit time. Scrap has to be avoided if possible. The process environment has to be free of particles, if possible, since particles quickly create scrap. Therefore semiconductor technology processes are typically performed in highest-grade clean rooms and the automation of the particular processes and of the steps between the processes in semiconductor production lines has reached a very high level.
For transportation between the various process stations, automated transportation mechanisms are provided, which for example among other things are being used for loading and unloading rapid heating systems which are being used for thermal treatment of substrates such as, e.g., semiconductor wafers. Rapid heating systems, also called RTP systems, are very well known in semiconductor production (U.S. Pat. No. 5,359,693 or U.S. Pat. No. 5,580,830). They are being used for thermal treatment of wafers consisting, for example, of silicon, but also of other semiconductor materials like Germanium, SiC, or other compound semiconductors like GaAs or InP. Such rapid heating systems must guarantee a yield of close to 100 percent. Therefore, the loading and unloading with the wafers to be processed is performed automatically by transportation mechanisms, which are also called wafer handlers. The wafer handlers have to be adjusted precisely relative to the systems they are serving in order to avoid possible damages to the material to be transported.
Transportation mechanisms in process equipment generally serve for transporting wafers or other semiconductor substrates from a wafer box into the process equipment and from the process equipment back into a wafer box. They typically have a rotation element rotatable around a rotation axis and at least one transport arm mounted to the rotation element. Often a lifting device is provided in order to allow transportation along the rotation axis. The transportation arm(s) also typically articulate in order to allow a lateral movement in addition to a rotation around the rotation axis. Therefore, a metal arm piece is typically mounted to one end of the rotation element and at its free end there is an additional metal arm piece, which is also rotatable around a second axis. While the rotation element is capable of performing rotations as well as up and down movements, the second rotation axis generally is a simple rotation axis which serves for adjusting the angle between the two arm pieces and thereby the distance of the substrate to be processed from the central rotation axis. At the end of the second arm piece generally an end piece with a carrier surface is mounted, which is provided for carrying the substrate.
In a piece of semiconductor production equipment, like e.g. a RTP system, this end piece, for example, has to fetch wafers out of a wafer box, load at least one process chamber with wafers, pick the wafers up from the process chamber after the process, possibly move the wafers into a second process chamber (e.g. a cooling chamber), remove them again from there, and put them into another wafer box. The particular stations to be approached one after another mostly are located around the transportation mechanism so that they can be reached by it quickly. Since the transportation mechanism is typically mounted onto a system platform in a solid manner it has to perform rotations around its central rotation axis in many cases in order to reach all stations. Therefore, the location of the central rotation axis relative to a predetermined direction can not be determined directly in many cases, since the rotation element is mounted into a housing by the manufacturer of the transportation mechanism and therefore is mostly not accessible for a direct measurement of its direction. However, the manufacturers of transportation mechanisms always strive to mount the transportation mechanism into a housing in a manner that at least one housing surface is perpendicular to the central rotation axis of the transportation mechanism and thereby can serve to align the central axis with a certain direction. Reality, however, shows that due to a manufacturer's tolerances the rotation axis is often not aligned in a certain direction with sufficient precision when it is aligned based on the direction of a housing surface. The alignment thereby becomes more difficult.
So far it has been attempted to solve the alignment problem with very imprecise methods, mostly guided by subjective impressions. Thus, it was attempted, e.g., to adjust for a horizontal position of the substrate carrier surface in a certain rotation position of the transportation mechanism with the consequence that the carrier surface was badly aligned in another rotation position, mostly in the opposite direction. This frequently caused either faulty handling or even damage to the substrates. In order to minimize the occurrence of errors, then, whole stations had to be realigned.
Another solution is described in U.S. Pat. No. 6,763,281. According to this Patent the alignment problem is solved through aligning the end piece of the transportation mechanism, which carries the material to be transported, relative to the system to be served through a specially developed alignment device. The alignment device has a body defining an opening, wherein the opening is large enough so the material to be transported can be moved through it. On the body around the opening a multitude of sensors is mounted. The alignment device has to be mounted onto a platform with defined directions relative to the system to be served. Thereafter the end piece is moved through the opening in order to determine its position relative to the system to be served through the sensors. This is a very complicated manual process since the alignment device has to be precisely mounted before performing the procedure. Furthermore, if the transportation mechanism serves several stations, all further stations have to be aligned to the end piece, which can be complicated.
If cassette stations are being aligned to fit the transportation mechanism, the cassette stations may end up tilting towards the inside, and the wafers housed in the cassette stations could then fall out due to gravity and system vibrations. Such vibrations can, e.g., already be created through the motions of the transportation mechanism. The fallen out wafers can break and thereby cannot be used anymore. System shutdown times for removing the broken wafers would have to be taken into consideration.
On the other hand, if a transportation mechanism has to be replaced with a new transportation mechanism, e.g. because it has reached the end of its useful life, all stations to be served have to be realigned accordingly, since the direction of the central axis of the new transportation mechanism does not necessarily coincide with the original direction of the central axis of the transportation mechanism, which was replaced.
Therefore, it is the objective of the present invention to determine the axis of a transportation mechanism relative to a reference axis, in particular relative to earth gravity, in a simple and cost effective manner. This determination allows the alignment of the rotation axis in a desired position, in particular in alignment with earth gravity.